Benzoxazines of the general formula
where p is 2, Y is selected from biphenyl, diphenyl methane, diphenyl isopropane, diphenyl sulfide, diphenyl sulfoxide, diphenyl sulfone, and diphenyl ketone, and R4 is selected from hydrogen, halogen, alkyl and alkenyl, are ordinarily prepared by reacting a phenolic compound, with an aldehyde compound and an alkyl amine. This reaction is known to be conducted under solventless conditions or in a solvent such as toluene, dioxane, alcohol or diethyl ether.
In U.S. Pat. No. 5,543,516 (Ishida) a method for preparing a benzoxazine compound is provided, in which a reaction mixture containing a phenolic compound, a primary amine, and an aldehyde is prepared without a separate solvent; the reactants are brought to a temperature at which the reactants combine chemically and are maintained at that temperature to reportedly form the benzoxazine compound. Ishida notes that prior art methods of forming benzoxazines require significant amounts of solvent to dissolve the reactants, leading to long reaction times. Ishida at col. 3, lines 43-52. Solventless systems require that at least one of the reactants be a liquid in order to solvate the reaction mixture. As not every reaction mixture will include a liquid reactant, solventless systems are not universally appropriate.
Chinese Patent Publication No. CN 1451679 (Gu) is directed to modified benzoxazine resins for RTM applications and discloses that in the past benzoxazine resins had been synthesized using organic solvents, such as toluene, dioxane and dimethyl benzene (citing to Chinese Patent Application No. CN 94111852.5). In the CN '679 publication, however, toluene is the only solvent used in the working examples.
Benzoxazines are presently available from several sources commercially, including Huntsman Specialty Chemicals, Brewster, New York; Georgia-Pacific Resins, Inc. and Shikoku Chemicals Corporation, Chiba, Japan, the last of which offers among others B-a, B-m, F-a, C-a and F-a benzoxazine resins.
However, these known synthetic methods and some commercial benzoxazines have shortcomings. For instance, some of the organic solvents used in the synthesis are toxic or environmentally hazardous and thus are not generally desirable for use in commercial processes. Indeed, many jurisdictions have implemented restrictions on the use of certain solvents and have even prohibited the use of particular solvents entirely. Others solvents, while deemed generally safe, must be removed using elevated temperatures that cause a degradation and/or premature polymerization of some benzoxazine compounds, resulting in compromised performance of curable compositions formulated with such benzoxazines.
The known synthetic methods usually require relatively long processing times, i.e., at least several hours, in order to perform the desired reaction, and to separate and purify the reaction products. Purification of the end product often requires additional time and is usually cumbersome. Notably, many common solvents used in the known commercial processes for preparing benzoxazimes pose toxicity risks, which may require expensive measures to eliminate, including the installation of costly solvent recovery systems or waste disposal.
Recently, Henkel Corporation (together with its parent corporation, Henkel AG & Co. KGaA, Dusseldorf, Germany) devised an improved synthesis for benzoxazines (See e.g. International Patent Application No. PCT/US2007/024519). That synthesis provides for: preparing a reaction mixture containing as reactants a phenolic component, a primary amine component, and an aldehyde component in an alkyl acetate solvent; and bringing the reactants and solvent to a temperature at which the reactants combine chemically and maintaining them at that temperature for a time sufficient to form a benzoxazine. And while this synthesis eliminated the need for the use of hazardous solvents such as toluene and dioxane, it would be desirable if even less volatile organic solvent could be employed to successfully produce benzoxazines.
A suitable reaction solvent should have at least some of the following properties. The reaction solvent should be inert to the reaction conditions and reagents, it should have an appropriate boiling point, and it should be easily removed at the end of the reaction. Solvents are usually classified into three general categories. “Polar protic” solvents which include water’ “dipolar aprotic” solvents that include ethyl acetate; and “non-polar” solvents that include toluene.
Moreover, a solvent for use in a benzoxazine-forming reaction requires special considerations. For example, the use of organic solvents with diamine starting materials is known to lead to the formation of solids due to poor solubility. Indeed, Ishida, in Polymer, 50 (2009) 5940-5944 recognized that “[t]he synthesis of aromatic diamine-based benzoxazines is generally hampered by the poor solubility of many aromatic diamines in the preferred solvents used for benzoxazine preparation. Also, the formation of stable triaza network structure resulted from the condensation of diamine and formaldehyde suppresses the reaction to continue for benzoxazine formation. In addition, the other side condensation reactions are quite possible. All these factors led to a difficulty for aromatic diamine-based benzoxazine preparation.”
The benzoxazine synthesis using toluene, a non-polar solvent, for instance, suffers from the high boiling point of toluene, even under vacuum. As a result, high temperature is needed to remove the toluene solvent from the benzoxazine product. However, since the benzoxazine product is somewhat volatile under these high temperature conditions, the toluene solvent cannot be completely separated from the benzoxazine product without removing the benzoxazine product as well. Moreover, the high temperatures required to remove the toluene also result in some degradation of the benzoxazine, along with some premature polymerization, ring opening, and increased viscosity.
The benzoxazine synthesis using ethyl acetate, a dipolar aprotic solvent, while an improvement over toluene, often shows the formation of undesirable solids, which may be removed by filtration, which can be undesirable. The filtrations can add time, labor, and cost, and also reduce the overall yield.
As such, an alternative solvent for the production of benzoxazines is desirable, whose properties include ease of product separation and low toxicity. The following solvent families have been determined to be unsuitable for the commercial synthesis of benzoxazines for the reasons given: